This invention relates to power field effect semiconductor devices, and more particularly, to field controlled semiconductor rectifiers having a field effect control structure monolithically integrated with the rectifier structure.
For power switching applications, in such electrical systems as motor drives and low to medium frequency (0-2000 Hz) power supplies, high speed and low loss performance at high current and high voltage levels is desirable. Prior art three terminal devices which can be used to control power delivered to a load include the MOSFET and the MOS gated thyristor. Prior art power MOSFET's include those described in U.S. Pat. No. 4,072,975, issued Feb. 7, 1978 to Ishitani and U.S. Pat. No. 4,145,703, issued Mar. 20, 1979 to Blanchard. Typical cross sections of power MOSFET devices are shown schematically in FIGS. 1 and 2 and their operating characteristics are shown in FIG. 3. These devices have been fabricated by using either planar diffusion techniques to form a DMOS structure 20 as shown in FIG. 1, or by etching V-grooves to form a VMOS structure 21 as shown in FIG. 2. In each case, for positive voltages applied to the drain, the junctions 22, 23 between the P-base regions 24, 25 and the N-drift regions 26 27 in FIGS. 1 and 2, respectively, block current flow between the drains 28, 29 and the sources 30, 31 in the absence of gate biases. Application of a sufficiently large positive gate bias with reference to the source results in the formation of an n-type inversion layer 32, 33 in the respective p-base regions under the gate electrodes 34, 35, respectively. This inversion layer allows conduction of electrical current from the drain to the source producing the forward conduction characteristics shown in FIG. 3. Increasing the gate bias, e.g., from V.sub.G1 through V.sub.G5, increases the conductivity of the inversion layer and thus allows higher drain current I.sub.Ds to flow. For negative voltages applied to the drain, the device conducts current like a forward biased p-n junction diode and cannot block current flow. As a result, these devices are operated with only positive voltages applied to the drain.
In MOSFET devices, only majority carrier (electron) current flow occurs between drain and source. This current flow is consequently limited by the majority carrier (electrons here) concentration in the channel and drift regions which determines their resistivity. For devices designed for operation at greater than 100 volts, the resistance of the drift region becomes large because the majority carrier concentration in the drift region must be small and the drift region width (W) must be large in order to support the device blocking voltages. Due to the high drift region resistance, high voltage MOSFET devices must be operated at low current densities to obtain low forward voltage drops. A typical current density of operation is about 50 A/cm.sup.2 at a forward voltage drop of 1.5 volts for a device capable of blocking up to 600 volts.
Despite this drawback of a high on-resistance, power MOSFET's have the advantage of requiring lower gate drive power levels than bipolar transistors since the gate voltage signal is applied across an insulating film. In these devices the drain current can also be turned off by bringing the gate voltage down to the source potential. This gate turn-off can be achieved with a higher current gain than for bipolar transistors.
Another type of prior art device is the MOS gated thyristor. Typical devices are disclosed in British Pat. No. 1,356,670, published June 12, 1974, U.S. Pat. No. 3,753,055, issued Aug. 14, 1973 to Yamashita et al., and U.S. Pat. No. 3,831,187, issued Aug. 20, 1974 to Neilson. A MOS gated thyristor is a pnpn thyristor structure, shown schematically in FIGS. 4 and 5, in which regenerative turn-on can be initiated by application of a voltage to an MOS gate. In the device 40 of FIG. 4, the MOS gate is formed on a surface 41 extending from the N+cathode 42 through the P-base 43 into a small portion of the N-base 44. In the device 50 of FIG. 5, the MOS gate is formed on a surface 51 extending along V-groove 52 from the N+cathode 53 through the P-base layer 54 into N-base 55. These devices will block current flow with either positive or negative voltages applied to their respective anodes 45, 56 in the absence of the gate bias. However, for positive anode voltages, the devices can be triggered into the conducting mode by application of a suitable positive voltage on the respective gates 46, 57. When a positive gate voltage is applied, the electric field across the gate oxide layers 47, 58 produces a depletion of carriers in the p-base under the gate electrode. As a result, the depletion layer in the p-base extends closer to the N+cathode region under the gate. This reduces the thickness of the undepleted p-base region of the upper NPN transistor under the gate electrode and thus increases its current gain. It is well known that a pnpn thyristor structure will switch from a current blocking state to a current conducting state when the sum of the current gains of the NPN and PNP transistors, .alpha..sub.NPN and .alpha..sub.PNP, respectively, exceeds unity. In the MOS gated thyristor, resistor increases until .alpha..sub.NPN +.alpha..sub.PNP exceeds unity. At this point strong injection of carriers must occur from the N+cathode into the p-base for the device to switch to the on-state. This requires that the N+P junction become forward biased by more than 0.5 volts. Once this takes place, the device switches to the conducting state and removal of the gate bias voltage will not cause the device to return to the blocking state because of the self-sustaining regenerative action inherent in the pnpn thyristor structure. Thus, these devices have the advantage of requiring low gate power to turn-on the thyristor via the MOS gate, but do not exhibit gate turn-off capability. Thus, the device must be returned to the blocking state by reversal of the anode polarity. The characteristics of the MOS gated thyristor are shown in FIG. 6, which show that these devices exhibit a negative resistance characteristic.